The invention concerns methods and devices for compensation of load factors in permanently excited motors wherein the rotor position is determined based on the inductivities of the phases.
Permanently excited synchronous machines (PMSM) and brushless direct current motors (BLDC) are comprised of a stator and a rotor with magnets. The individual phases of the stator are connected in a star connection or delta connection. The motor is operated with a converter. These motors require control electronics for determining the rotor position and for supplying current to the individual phases.
The rotor position can be detected by means of sensors. Inter alia, Hall sensors are employed. Sensors require mounting space and cause higher system costs.
In order to avoid these disadvantages, control methods without sensors are employed. They can be classified roughly into two groups.
One group employs the voltage that is induced by movement within the phases. The system-caused disadvantage resides in the lack of positional information at standstill.
The second group is based on the variation of the stator inductivities. The saturation of the stator inductivity is affected by the rotor field of the permanent magnets and the current-caused stator field.
The inductivity of a coil in the currentless state is proportional to the relative permeability. As a result of the saturation effects that occur in the core, with increasing field strength the relative permeability drops or, stated differently, the magnetic resistance increases. In this connection, only the absolute value but not the direction is decisive. Thus, a rotor position-dependent function of the inductivity for a BLDC motor results. When the motor has three phases, the functions are displaced relative to each other by 60 electrical degrees. In the currentless state, the north and the south poles of the rotor magnets have the same effect. Therefore, these functions of the inductivities have twice the periodicity relative to the electrical period. This ambiguity must be resolved for a complete position information. A current-excited field is superimposed on the stator field. In this way, the saturation in the motor is either enlarged or reduced and the corresponding inductivity is thus reduced or enlarged. This effect affects also the angle determination in case of useful current feed of the motor and causes a faulty determination of the rotor position.
The invention has the object to stabilize the inductivity-based signals for positional determination of the rotor in permanently excited motors with respect to load factors.